I'd send a nice letter thanking the lady for her kind comments,
and enclose the bill for the amount she had agreed to.
Then if you don't get it, well, both she and her daughter are taking advantage of you. I had one like that many years ago, over 50 years ago, and I'm still bitter when I think of what she did.
Clear out the air with her, and start looking for either a new apartment or a new room mate. This might escalate once they know they can get away with it. Take any small, valuable items to a bank lock box, and take pictures of all the rooms that are yours, keep bills of sale in the bank also. This I'm saying from experience.